1. Technical Field of the Invention
The present invention relates to an image display device which is capable of individually displaying images that are to be viewed from a plurality of view points and displaying an image without reducing resolution when images different from one another are viewed from the plurality of view points, a portable terminal device incorporating therein such image display device, a display panel incorporated within the image display device, and an image display method. Particularly, the present invention relates to an image display device capable of displaying a three-dimensional image without reducing resolution, displaying a two-dimensional image and three-dimensional image with the same resolution, and further, displaying a two-dimensional image and three-dimensional image at any location in a blended fashion, a portable terminal device incorporating therein such image display device, a display panel incorporated within the image display device, and an image display method using the same.
2. Description of the Related Art
Conventionally, a display device has been studied which is capable of displaying a three-dimensional image. In 280 years B.C., Greece-mathematician Euclid considered and defined “binocular vision is what a person perceives when he/she sees, using his/her right and left eyes one at a time, different images of the same object that are created when viewed from different directions” (non-patent literature 1: Chihiro Masuda, “Three-Dimensional Display” Sangyo Tosho, K.K.). That is, a three-dimensional display device should have a capability to distribute images with parallax to right and left eyes, respectively.
A number of three-dimensional image display methods have conventionally been studied and developed as a method for realizing the above-stated capability and those methods can be classified mainly into two types of methods, i.e., a method using eyeglasses and a method not using eyeglasses. One of methods of the type using eyeglasses would be an anaglyph method using difference in color, a polarized eyeglasses method using polarization, or the like. However, in the method using eyeglasses, a user of eyeglasses basically cannot remove the burden of wearing eyeglasses and in consideration of such problems, non-eyeglasses observing, which is performed by not using eyeglasses, has intensely been studied and developed in recent years. The non-eyeglasses observing includes, for example, a method using a lenticular lens and a method using a parallax barrier.
The parallax barrier method was conceived by Berthier in 1896 and identified by Ives as practical in 1903. FIG. 1 shows an optical model illustrating a three-dimensional image display method using a parallax barrier. As shown in FIG. 1, a parallax barrier 105 is a barrier (light shield) which has a number of fine vertical-striped openings, i.e., slits 105a formed therein. Furthermore, disposed in the vicinity of one surface of the parallax barrier 105 is a display panel 106. In the display panel 106, pixels 123 for right eye and pixels 124 for left eye are arranged in a direction orthogonal to the longitudinal direction of the slits. Moreover, disposed in the vicinity of the other surface of the parallax barrier 105, i.e., on the side opposite the display panel 106 is a light source 108.
Lights were emitted from the light source 108 and have transmitted through the openings (slits 105a) of the parallax barrier 105 and then through the pixels 123 for right eye, thereby flowing out as light fluxes 181. Similarly, lights were emitted from the light source 108 and have transmitted through the slits 105a and then through the pixels 124 for left eye, thereby flowing out as light fluxes 182. In this case, the location of an observer who is able to identify a three-dimensional image is determined by a positional relationship between the parallax barrier 105 and the pixels. That is, it is required that a right eye 141 of the observer 104 falls within an area through which all of the light fluxes 181 corresponding to a plurality of pixels 123 for right eye pass and a left eye 142 of the observer 104 falls within an area through which all of the light fluxes 182 corresponding to a plurality of pixels 124 for left eye pass. As shown in FIG. 1, this positional relationship between the eyes and the light fluxes corresponds to the case where a midpoint 143 between the right eye 141 and left eye 142 of the observer falls within a quadrangle shaped three-dimensional visible range 107 shown in FIG. 1. Among line segments extending in a direction along which the pixels 123 for right eye and pixels 124 for left eye are arranged in the three-dimensional visible range 107, a line segment passing through a intersection 107a of diagonal lines in the three-dimensional visible range 107 is longest. Accordingly, in case of the midpoint 143 being positioned at the intersection 107a, a latitude of a displacement which is allowed when the position of the observer is displaced in a right or left direction becomes maximum and therefore, when the midpoint 143 is positioned at the intersection 107a, it can be concluded that the observer views images from a most preferable position. In consideration of the above-described fact, it is recommended that the three-dimensional image display method is constructed such that when assuming a distance between the intersection 107a and the display panel 106 is an optimal observation distance OD, the observer views images keeping the distance OD from the display panel. Note that a virtual plane spaced apart from the display panel 106 the optimal observation distance OD in the three-dimensional visible range 107 is referred to as an optimal observation plane 107b. This allows lights from the pixels 123 for right eye and the pixels 124 for left eye to reach the right eye 141 and left eye 142 of the observer, respectively. Therefore, it becomes possible that the observer identifies an image displayed on the display panel 106 as a three-dimensional image.
At the beginning of emergence of the method using a parallax barrier, the parallax barrier was disposed between pixels and eyes, and therefore, a problem arises in that the parallax barrier obstructs the view and serves to cause the visibility of an image to be displayed to be low. However, the recent commercialization of a liquid crystal display panel makes it possible to dispose the parallax barrier 105 on the rear side of the display panel 106 to improve the visibility of an image to be displayed. This currently leads to enhancement of the study and development of a three-dimensional image display device using a parallax barrier.
An example of a product which uses a parallax barrier and became a commercial reality is described in a table 1 of Nikkei Electronics, No. 838, pp. 26-27 issued on Jan. 6, 2003 (non-patent literature 2). This product is a cellular phone incorporating therein a 3D (three-dimensional) liquid crystal display panel and the liquid crystal display panel making up a three-dimensional image display device is 2.2-inch diagonal in size and has 176 columns of 220 dots as a display dot. Furthermore, a liquid crystal display panel for switching between on and off of effect of parallax barrier is provided allowing a display on the panel to switch between three-dimensional display and two-dimensional display. Although the display device displays a two-dimensional image with a definition of 128 dpi in both vertical and horizontal directions, at the time of display of three-dimensional image, the display device displays images for left eye and images for right eye in a vertical stripe form and in an alternate fashion, and therefore, the display device displays the image with a definition of 64 dpi in a horizontal direction, which definition is half the definition, 128 dpi, in a vertical direction.
Furthermore, the method using a lenticular lens is invented by Ives et al. in around 1910, as described in, for example, the aforementioned non-patent literature 1. FIG. 2 is a perspective view illustrating a lenticular lens and FIG. 3 shows an optical model illustrating a three-dimensional display method using a lenticular lens. As shown in FIG. 2, a lenticular lens 121 is constructed such that one surface of the lens is planarized to provide a plane and the other surface thereof has formed therein a plurality of hog-backed projections (cylindrical lens 122), each extending in one direction, so that the longitudinal directions of the projections are parallel to one another.
Moreover, as shown in FIG. 3, a three-dimensional image display device of the type using a lenticular lens is configured so that a lenticular lens 121, a display panel 106 and a light source 108 are arranged in this order when viewed by an observer, and pixels of the display panel 106 are positioned on a focal plane of the lenticular lens 121. The display panel 106 is constructed such that pixels 123 for display of an image for right eye 141 and pixels 124 for display of an image for left eye 142 are arranged in an alternate fashion. In this case, individual sets of the pixel 123 and pixel 124, those pixels being adjacent each other, are provided so as to correspond to the cylindrical lenses (projections) 122 of the lenticular lens 121, respectively. This allows light emitted from the light source 108 and having transmitted through the individual pixels to be distributed by the cylindrical lenses 122 of the lenticular lens 121 in directions toward left and right eyes. Thus, it becomes possible for the left and right eyes to identify images different from each other, thereby allowing an observer to identify a three-dimensional image.
Whereas the method using a parallax barrier is the method for “shielding” unnecessary light rays by using a barrier, the method using a lenticular lens is the method for changing a direction in which light propagates and therefore it can be concluded that providing the lenticular lens theoretically never reduces the brightness of display screen. Accordingly, application of the method using a lenticular lens to a portable device etc. whose ability to display images with high brightness and operate with low power is regarded as particularly important is promising.
An example of a three-dimensional image display device developed using a lenticular lens is described in the above-stated non-patent literature 2. A liquid crystal display panel making up a three-dimensional image display device is 7-inch diagonal in size and has 800 columns of 480 dots as a display dot. Furthermore, changing a distance between a lenticular lens and a liquid crystal display panel by 0.6 mm allows switching between three-dimensional display and two-dimensional display. The number of view points arranged in a horizontal direction is five and when an observer changes his/her viewing angle in a horizontal direction, he/she can view five different images. That is, a display definition achieved at the time of display of three-dimensional image is reduced to one fifth of a display definition achieved at the time of display of two-dimensional image.
Furthermore, a simultaneous multiple-image display for simultaneously displaying multiple images has been developed as an image display device using a lenticular lens (for example, refer to the patent literature 1: Japanese Patent Laid-Open Publication No. H06(1994)-332354 (FIG. 13)). This display is configured so that through use of lenticular lens ability to distribute images, images different from one another when viewed from different directions are simultaneously displayed under the same conditions. This allows a single simultaneous multiple-image display to simultaneously provide a plurality of observers, who are positioned in directions different from one another relative to the display, with images different from one another. The patent literature 1 discloses that when using the simultaneous multiple-image display, saving of space for arrangement of displays and reduction in electricity expense are achieved, but those beneficial effects cannot be obtained in the case where a number of displays corresponding to the number of observers are prepared.
However, the aforementioned conventional techniques have the following problems. That is, when displaying images different from one another so that the images are viewed from a plurality of view points, the resolution of individual images to be displayed is disadvantageously reduced. Particularly, the resolution of an image to be displayed is reduced to a greater extent at the time of display of three-dimensional image than at the time of display of two-dimensional image. FIG. 4 is a top view illustrating sub-pixels in the aforementioned three-dimensional image display device using the parallax barrier and allowing view from two view points. One display pixel to be used at the time of display of three-dimensional image is comprised of two display pixels to be used at the time of display of two-dimensional image. At the time of display of three-dimensional image, the two display pixels serve as a pixel for left eye and a pixel for right eye to allow the display device to display an image for left eye and an image for right eye, respectively. The pixel for left eye and the pixel for right eye each are comprised of three primary color sub-pixels with primary colors, red, blue and green, and three slit openings correspond to one display pixel. In more detail, the red sub-pixel 411 for left eye and the green sub-pixel 422 for right eye correspond to a first slit opening. Furthermore, the blue sub-pixel 413 for left eye and the red sub-pixel 421 for right eye correspond to a second slit opening. Still furthermore, the green sub-pixel 412 for left eye and the blue sub-pixel 423 for right eye correspond to a third slit opening. Note that the individual sub-pixels are partitioned by a light shield section 6. When assuming a pitch of primary color sub-pixels arranged in the longitudinal direction (vertical direction 11) of slit opening is a and a pitch of the primary color sub-pixels arranged in a direction (horizontal direction 12) orthogonal to the longitudinal direction of slit opening is b, the following expression 1 results.a:b=3:1  (Expression 1)
As a result, a relationship between a pitch a of display pixels arranged in the longitudinal direction of slit opening and a pitch c of the display pixels arranged in a direction orthogonal to the longitudinal direction of slit opening is represented by the following expression 2. That is, when a three-dimensional image is displayed by the three-dimensional image display device shown in FIG. 4, the size of one display pixel is represented by a in the longitudinal direction of slit opening and by c in the direction orthogonal to the longitudinal direction.a:c=1:2  (Expression 2)
On the other hand, when a two-dimensional image is displayed by the three-dimensional image display device shown in FIG. 4, the parallax barrier 105 is removed and the one display pixel to be used at the time of display of three-dimensional image is used as two display pixels. Note that a method for removing a parallax barrier includes, for example, constructing a parallax barrier by a liquid crystal display panel for switching between on and off of effect of parallax barrier, as shown in the aforementioned non-patent literature 2, and changing light transmittance of individual elements of the liquid crystal display panel. Furthermore, when a lenticular lens is used instead of parallax barrier, changing a distance between the display panel and the lenticular lens allows elimination of effects of the lenticular lens.
In more detail, at the time of display of two-dimensional image, as shown in FIG. 4, three sub-pixels, i.e., the red sub-pixel 411 for left eye, green sub-pixel 422 for right eye and blue sub-pixel 413 for left eye, are used as one display pixel and three sub-pixels, i.e., the red sub-pixel 421 for right eye, green sub-pixel 412 for left eye and blue sub-pixel 423 for right eye, are used as one display pixel. As a result, the size of one display pixel is represented by a in the longitudinal direction of slit opening and by (c/2) in the direction orthogonal to the longitudinal direction. Therefore, it turns out that the pitch, used at the time of display of three-dimensional image, of the pixels arranged in the direction orthogonal to the longitudinal direction of slit opening becomes twice the pitch, used at the time of display of two-dimensional image, of the pixels arranged in the same direction. Accordingly, as is the case with the three-dimensional image display device described in the aforementioned non-patent literature 1, the resolution of an image to be displayed at the time of display of three-dimensional image in the horizontal direction 12 is reduced to half the resolution of an image to be displayed at the time of display of two-dimensional image.
The reduction in the resolution of an image to be displayed becomes problematic particularly when a three-dimensional image is displayed together with character information and when character information is three-dimensionally displayed. Since the display pixel is caused to take a shape of a rectangle with a 1:2 aspect ratio, the resolution in a horizontal direction is reduced and lack is generated in a vertical line making up a character when the character is displayed. As a result, the visibility of a character to be displayed is reduced to a large extent. This problem becomes more prominent as the number of view points increases.
Problems similar to those described above are not limited to a three-dimensional image display device, but generally observed in the display device for displaying images so that the images are viewed from a plurality of view points. That is, when displaying images so that the images are viewed from a plurality of view points different from one another, the resolution of images arranged in a direction in which sub-pixels that are to be viewed from a plurality of view points are arranged is reduced to an extent larger than the resolution achieved at the time of display of a single image and especially when a character is displayed together with the images to be viewed from a plurality of view points, the visibility of character is disadvantageously and significantly reduced.
Furthermore, a problem arises in that when using the conventional techniques relating to the aforementioned three-dimensional image display device, switching between three-dimensional display and two-dimensional display is performed all over the screen and therefore it is impossible that a three-dimensional image and a two-dimensional image are displayed on any location in a blended fashion.